The present invention relates generally to gas lasers and, more particularly, to a circuit for facilitating the start-up of a radio frequency driven laser by ionizing a portion of the gain medium of the laser.
A carbon dioxide (CO.sub.2) radio frequency (RF) driven laser is typically used as a source of coherent radiation in systems such as laser sensor systems. Such sensor systems are frequently found on vehicles where human-attended operation is not possible, such as a missile or a satellite. Failure to start the laser in such a sensor system would result in complete failure for the entire system and possibly the mission. Thus, a highly reliable means for starting an RF laser may be of vital importance in such applications.
When two electrodes having a gas at low pressure between them are subjected to an increasing electrical potential, an initial, very small current will flow as the charged particles present in the gain medium gas are attracted to the electrodes. As the potential increases, the current increases rapidly as the electrons which are attracted to the positive electrode gain sufficient energy to ionize the gain medium atoms and thereby generate more current carriers. Suddenly, the current increases very rapidly and the voltage between the electrode drops as, under proper conditions, the gas achieves a self-maintaining discharge called a glow.
A discharge current may be maintained in a rarefied gas without the introduction of the electrodes into the gas. A tube of gain medium gas may be placed between two electrodes having a rapidly-alternating potential difference. The gas within the tube acts as a capacitor dielectric, and the gain medium may become luminous as a result of a discharge across the tube, similar to that of the discharge across internal electrodes.
RF lasers typically require much more RF driver power to initially ionize their internal gas mixtures than is required to maintain a steady state gas discharge under normal operating conditions. In some prior art gas laser systems, small amounts of xenon are added to the gas mixture to lower the potential required to ionize the gas medium. The high starting power requirement can be made even more severe by environmental temperature extremes or long time periods of inactivity of the laser.
Laboratory experience has shown that a difficult-to-start laser can usually be started if its internal gas mixture is stimulated by an electric discharge from a Tesla coil before RF power is applied. However, the inclusion of a conventional Tesla coil adds size, weight, complexity and expense to a laser system, and it may require an additional energy source to activate the coil.